1. Field of the Invention
The present invention relates to a diversity receiving apparatus suitable for a vehicle-mounted receiver for terrestrial digital TV broadcasting that is driven using an OFDM (Orthogonal Frequency Division Multiplex) method.
2. Description of the Related Art
FIG. 2 shows a conventional diversity receiving apparatus in which a gain synthesizing method is used to maximize a C/N ratio after diversity synthesis. In FIG. 2, one receiving system comprises an antenna 21 and a receiver 22 connected to the antenna 21, and the other receiving system comprises an antenna 23 and a receiver 24 connected to the antenna 23.
An adder 25 adds a reception signal output from the receiver 22 to a reception signal output from the receiver 24 to synthesize them, and a phase shifter 26 is provided between the receiver 24 and the adder 25. In addition, a phase difference detector 27 detects the phase difference between the reception signal output from the receiver 22 and the reception signal output from the receiver 24 and thus the phase shifter 26 is controlled based on the detected results.
According to this configuration, when a phase difference exists between two reception signals, the phase difference detector 27 controls the phase shifter 26 to remove the phase difference. As a result, two reception signals to be input into the adder 25 are controlled to have the same phases, and thus the signal synthesized by the adder 25 has the maximum C/N ratio (for example, see Non-Patent Document 1).
[Non-Patent Document 1]
‘Modulation and Demodulation in Digital Wireless Communication’ written by Yoichi Saito and published by the Institute of Electronics, Information and Communication Engineers of Japan (IEICE) on Feb. 10, 1996, pp. 189 to 191 and FIG. 5.19
The reception signal synthesized by the adder 25 is converted into a base-band signal by OFDM demodulating means (not shown).
According to the aforementioned method, for example, when the antenna 21 receives a signal having a low C/N ratio that mainly includes noise components and the antenna 22 receives a signal having a high C/N ratio that mainly includes signal components, phase of the signal having a high C/N ratio is controlled so as to match to the phase of the signal having a low C/N ratio. Therefore, in an extreme case, the phase of the signal having a high C/N ratio is continuously varied for phase matching, and thus it is difficult to improve a C/N ratio of the synthesized signal and to synchronize signals.